If you are able to machine your own axles according to your needs, any stock hub with industrial bearings (no- cones).

For drum brakes: Sturmey Archer, as on most stock trikes with drum brakes. Find a trike dealer that is willing to sell you the wheels and kingpins designed to go with them. here in Europe a ready- to- install 20 inch trike front wheel runs about 65 Euros, to give you a fant idea.

I noticed that the bikepartsusa website sells Sun rear 20x1.75 trike wheels for $28.78, but they lack fancy hub/disk brakes:http://www.bikepartsusa.com/cgi-bin/...=wheel-20_inch
-with 36 spokes (so you could find other rims if you wanted different ones) and 15mm/35mm bearings.

I'm assuming this is the wheel for the free-spinning side; I have no idea how Sun drives the rear wheel of their trike. They may use the same hub for both sides somehow.

The rim widths on the pre-built 20" rims there are 1.5", 1.75" and 2". I chose a couple regular front wheels with the 14mm axles, to do it the fleetrikes way--but in almost all of these wheels, you are stuck with 48-spokes. For a non-tilting trike that's not any complaint, but for a tilting trike that won't stress the wheels much laterally it's kinda overkill.
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I got a pair of BMX/freestyle wheels, as per the fleetrikes site. After shifting the axles as far to one side as they'd go, there is 1.5 inches on the "long" side.

There's still about 1/4" on the other side, but the bearing cones would not spin past the center of the axle. I didn't take the whole thing apart to see why, but it appears the axles aren't threaded in the very center for some reason.

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By the by--what sort of rake/trail is typically used for trikes with 406/20" front wheels? The few commercial examples I've seen seemed to have no trail at all...

I even looked for rake-trail info for BMX bikes, and couldn't find any. Front-end geometry is normally provided for road/MTB's, but the BMX companies don't seem to bother with publishing it (though I see that the head tube angles look pretty steep anyway). I had wondered about building it with adjustable kingpin angle, since I doubt I'll get it perfect trying to weld both sides in place anyway....
~

I got a pair of BMX/freestyle wheels, as per the fleetrikes site. After shifting the axles as far to one side as they'd go, there is 1.5 inches on the "long" side.

There's still about 1/4" on the other side, but the bearing cones would not spin past the center of the axle. I didn't take the whole thing apart to see why, but it appears the axles aren't threaded in the very center for some reason.

----

By the by--what sort of rake/trail is typically used for trikes with 406/20" front wheels? The few commercial examples I've seen seemed to have no trail at all...

I even looked for rake-trail info for BMX bikes, and couldn't find any. Front-end geometry is normally provided for road/MTB's, but the BMX companies don't seem to bother with publishing it (though I see that the head tube angles look pretty steep anyway). I had wondered about building it with adjustable kingpin angle, since I doubt I'll get it perfect trying to weld both sides in place anyway....
~

With regard to the axles, you're right. The BMX axles have a center section that is unthreaded.

When I use the BMX wheels, I use 15º of king pin inclination and 15º of caster. The king pin inclination angle may need to be tweaked depending on how you build your axle tubes, but it'll get you close. Just make a little jig and you'll get the same angles every time.http://fleettrikes.com/tth2.htm#bushings